Microdialysis probe technology provides access to tissue interstitium for either sampling of diffusible tissue constituents or for delivery of bioactive substances; however, the relationship of the probe perfusate concentrations to their tissue counterparts is a complex function of solute molecular weight, solute physicochemical properties, tissue properties, probe membrane properties, probe geometry and perfusion rate, and the trauma of probe insertion into the tissue. These dependencies are being studied in order to improve the quantitative usefulness of the technology. The focus of applications is the brain in connection with diseases such as Parkinson's, AIDS, and Alzheimer's. Endogenous solutes of interest include dopamine and its metabolites. Exogenous substances employed as marker solutes or pharmacological agents are acetaminophen, AZT, and sucrose. Mathematical modeling is being used to describe solute transport within the probe and surrounding medium. validation experiments involve tissue autoradiography and histology, as well as measurement of perfusate concentrations.